Seventy five Syrian rebels trained by the United States and its allies to fight Islamic State have entered northern Syria since Friday, September 18, the Syrian Observatory for Human Rights said on Sunday, according to Reuters.

Rami Abdulrahman, director of the Observatory, said the rebels had crossed into Syria from Turkey with 12 vehicles equipped with machine guns.

A top U.S. general told Congress on Wednesday that only four or five Syrian rebels trained under the programme aimed at fighting Islamic State were still fighting in Syria.

The U.S. military began training in May for up to 5,400 fighters a year, in what was seen as a test of President Barack Obama's strategy of having local partners combat Islamic State militants and keep U.S. troops off the front lines.